Field of the Invention
The present invention relates to a rotor for a permanent magnet electric motor, to a method of fabricating such a rotor, and to an electric motor including such a rotor.
Electric motor rotors are known that comprise a metal magnetic core with permanent magnets fastened to its periphery. By way of example, the permanent magnets may be adhesively bonded to the magnetic core.
In order to prevent the forces to which the magnets are subjected, in particular as a result of centrifugal force, causing the magnets to be torn from the magnetic core, it is known to cover the magnets in a steel sleeve, also referred to as a band, pressed tightly against the magnetic core and the magnets. In order to guarantee that the sleeve is sufficiently tight, it is necessary to true the rotor and the magnets, and to machine the inside diameter of the sleeve accurately. In addition, since the thickness of the sleeve determines the size of the magnetic gap between the rotor and the stator, it is necessary to adjust the outside diameter of the sleeve by machining once the sleeve is in place on the magnetic core and the magnets.
Description of Related Art
U.S. Pat. No. 4,930,201 explains how such a sleeve presents the drawbacks of increasing the inertia of the rotor and of facilitating the flow of eddy currents in the motor. That document proposes replacing the steel sleeve with a sleeve made as a winding of a thread of glass, aramid, or epoxy fibers, which winding is embedded in a resin. According to that document, the sleeve as formed in that way presents several advantages:                it presents smaller inertia than the sleeve made of steel;        it does not conduct and therefore it does not allow eddy currents to flow; and        the sleeve has a coefficient of thermal expansion that is smaller than that of the magnetic core such that heating of the rotor increases the clamping force exerted by the sleeve on the magnetic core.        
Nevertheless, it is found that that arrangement suffers from the drawback of being difficult to perform. The thread is pre-impregnated with resin in liquid or viscous form, thereby making the thread difficult to handle. It is also possible to put the resin into place after winding the thread, but that adds an additional step and makes it necessary to ensure that the resin is uniformly distributed over the entire surface area and thickness of the sleeve.